The invention relates to an angle sensor comprising at least two angle-offset anisotropic magnetoresistive sensor units (AMR sensor unit) comprising anisotropic magnetoresistive elements (AMR elements). Such AMR sensors for angle measurement are used in different areas, often in automobile manufacture, in which such sensors are used, for example, as choke-angle sensors in combustion engines or steering angle recognition.
Such an angle sensor is known from German Offenlegungsschrift DE44 08 078A1.
When such angle sensors are used in a magnetic field which is clearly stronger than its anisotropic field strength, which is the typical field of use of the known AMR angle sensors, the sensor is driven into saturation so that it generates a characteristic with a sin (2xcex1) periodical variation.
The sin (2xcex1) periodical characteristic has the consequence that the angle sensor can only perform 180xc2x0 angle measurements because, due to the periodicity, it is not possible to distinguish between 0xc2x0 and 180xc2x0.
For a 360xc2x0 angle measurement at higher field strengths, more complex sensors such as, for example, GMR sensors (giant magnetoresistive sensors) or technological systems with a plurality of sensors and toothed wheels are required, which leads either to a highly complex manufacture and an attendant increase of costs, or to the fact that the angle sensors can no longer operate without contact. Moreover, the long-term stability of GMR sensors is currently not quite clear.
It is therefore an object of the invention to provide an AMR angle sensor which can be manufactured in a simple and low-cost way and can also cover a range of 360xc2x0, even at high field strengths, as may particularly occur in automobiles.
This object is solved by an angle sensor which comprises at least two angle-offset anisotropic magnetoresistive sensor units comprising anisotropic magnetoresistive elements, in which the angle sensor comprises at least a device for increasing the anisotropic field strength of the sensor units, and by an angle sensor in which the magnetoresistive element comprises magnetoresistive layers, particularly magnetoresistive strips which have a substantially hyperboloid or asteroidal shape. The object is further solved by a method of increasing the anisotropic field strength of a sensor unit of an angle sensor comprising magnetoresistive elements, in which a magnetic supporting field present in the preferred direction of the magnetoresistive elements is generated, and by a method in which the magnetoresistive elements are built up of magnetoresistive layers, particularly magnetoresistive strips, in which the magnetoresistive layers are given a substantially hyperboloid or asteroidal shape.
According to the invention, the angle sensor comprises at least a device for increasing the anisotropic field strength of the sensor units used in the angle sensor. An angle sensor according to the invention utilizes the fact that the characteristic of an AMR angle sensor has a periodicity of sin (xcex1) when the sensor is driven at field strengths below the anisotropic field strength. At such a periodicity, a 360xc2x0 measurement is also possible.
Due to the use of an angle sensor with a device for increasing the anisotropic field strength of the sensor units, the anisotropic field strength can therefore be increased so far that even the high field strengths occurring particularly in automobiles but also in other areas are below the anisotropic field strength of the AMR angle sensor. Consequently, an AMR angle sensor is provided in a simple and low-cost way which renders a 360xc2x0 measurement possible in a low-cost and contactless way, even at very high field strengths.
In a preferred embodiment, the device for increasing the anisotropic field strength of the sensor unit impresses a supporting field in a preferred direction of the magnetoresistive elements, which field increases the anisotropic field strength of the sensor units and the magnetoresistive elements (MR elements).
Typical field strengths for a supporting field in the preferred direction are between 1 and 10 kA/m, preferably between 1 and 6 kA/m, in which conventional magnetic field strengths are at about 4 kA/m. The measuring field strengths in which the angle sensor is used are of a similar order of magnitude, but the measuring field strength is preferably slightly smaller than the field strength of the supporting field. Typical measuring field strengths are in a range of 3 kA/m but are quite dependent on the fields in which they are used. In a preferred embodiment, a soft magnetic shield against external interference fields is advantageous.
The MR elements generally comprise parallel arranged MR strips which are aligned in a preferred direction but may also be formed differently.
In accordance with a preferred embodiment of the angle sensor according to the invention, the device for increasing the anisotropic field strength comprises at least a coil arrangement in which at least one external coil per magnetoresistive element is preferably used, which coil substantially completely surrounds the MR element so that, in the core area of the coil, the magnetic field is aligned in the preferred direction of the MR elements.
Instead of, or in addition to the external coils, another embodiment provides the possibility of using at least one thin-film coil integrated in the layout of each MR element.
Further possibilities of increasing the anisotropic field strength of the sensor units in accordance with the invention are in the field of using at least one magnet whose magnetic field is aligned in the preferred direction of the MR elements, for which both permanent magnets and electromagnets can be used. Permanent magnets are, however, preferred because they have less influence on the angle sensor during use.
It is to be noted that the devices for increasing the anisotropic field strength of the sensor units may be devices which permanently increase the anisotropic field strength such as, for example, the above-mentioned permanent magnet, while the invention also encompasses devices which can increase the anisotropic field strength in a discontinuous and a controlled manner as in, for example, the above-mentioned coil arrangements or in the case of electromagnets. In this case, the anisotropic field strength may be controlled temporally and as regards its extent, while the invention encompasses both a periodical and also a non-periodical increase of the anisotropic field strength in the case of temporal control.
A further possibility for the device for increasing the anisotropic field strength is a hard magnetic layer which is provided on the MR elements. This device is a device which permanently increases the anisotropic field strength.
In a particularly preferred embodiment, the at least two angle-offset anisotropic magnetoresistive sensor units are tilted by an angle of 90xc2x0, which provides the possibility of a 360xc2x0 measurement. In this respect it is to be noted that also a tilt of, for example, about 45xc2x0 is possible and that the angle sensor according to the invention can be designed with the device for increasing the anisotropic field strength of the sensor units in dependence upon their field of use and in such a way that it only measures a reduced angle range of, for example, 180xc2x0. The angle sensor according to the invention is therefore not limited to a 360xc2x0 AMR angle sensor, although this is a preferred field of use and the advantages of the angle sensor according to the invention become particularly manifest in such a measurement.
The MR elements in the sensor units preferably comprise MR layers, particularly MR strips having a predetermined length (l), width (b) and thickness (d). To further support an increase of the anisotropic field strength, it is particularly advantageous to have a possibly large ratio between the thickness (d) and the width (b), while ratios V=d/b are particularly preferred that exceed values of 4*10xe2x88x923, particularly a ratio Vxe2x89xa71.5*10xe2x88x922.
It is further preferred to form at least one of the MR layers rectangularly, hyperbolically and/or asteroidally, so that an increase of the anisotropic field strength of the sensor element is also achieved.
It is further preferred that the MR layers consist of a material having a high intrinsic material anisotropy. In this respect, preferred materials are particularly NiCo 50:50 (HK=2500), NiCo 70:30 (HK=2500), CoFeB 72:8:20 (HK=2000), NiFe 81:19 (HK=250) in which HK is the intrinsic material anisotropy.
The choice of the ratio between the thickness and width of the MR layers as well as the choice of the geometrical shape of the MR layers and the material of the MR layers enhance the effect of the above-described devices for increasing the anisotropic field strength of the sensor units but they also increase the anisotropic field strength of the sensor units independently of the devices described above and are therefore to be considered as independent inventive aspects.
In accordance with a further independent inventive aspect, the angle sensor further comprises means for applying magnetic reversal coils, which means preferably comprise at least one coil arrangement. The external coils, or the internal thin-film coils integrated in the layout of the MR elements described above are also suitable as coil arrangements.
The magnetic reversal coils are at least used in a controlled manner at given points, where xe2x80x9cflippingxe2x80x9d of the characteristic curve is compensated, which occurs when the angle sensor is used in the range of anisotropic field strengths and moves in a limit range in which it generates characteristics with a sin (2xcex1) periodicity and characteristics with a sin (xcex1) periodicity, which leads to a spontaneous magnetic reversal in MR sensor elements.
The angle sensor according to the invention therefore provides the possibility of a 360xc2x0 angle detection in that the anisotropic field strength is increased to such an extent that the field strength in which the sensor is used is clearly below the anisotropic field strength of the sensor, so that a characteristic with a sin (xcex1) periodicity is generated, or in that the spontaneous reversal of magnetism (flipping of the characteristic curve) occurring in the limit range between the sin (2xcex1) and the sin (xcex1) periodicity is clearly defined by magnetic reversal coils so that a 360xc2x0 signal is obtained after signal evaluation.
Dependent on the field of application, both measures can be used both jointly and separately.
A further embodiment of the angle sensor comprises a subsequently arranged or integrated signal electronic circuit which generates a post-correction of the output signals, particularly a correction which becomes necessary because of the above-described flipping of the characteristic curve so that also in this case a 360xc2x0 signal is obtained, even when the means for applying magnetic reversal coils are not provided and the anisotropic field strength of the sensor unit is not clearly above the field strength in which the angle sensor is used, so that also this signal electronic circuit is an independent inventive aspect.
The invention further relates to a method of increasing the anisotropic field strength of a sensor unit of an angle sensor comprising magnetoresistive elements having a preferred direction, in which in accordance with the inventive method a magnetic supporting field in the preferred direction is generated. An AMR angle sensor can thereby be provided in a simple manner, which renders a 360xc2x0 measurement possible, also at high field strengths. These and further advantages correspond to the advantages which have been explained with reference to the angle sensor according to the invention.
The supporting field may be applied both permanently and discontinuously, in which the above-mentioned devices and methods can be used for generating the supporting field. It is particularly preferred that the supporting field is generated by means of a coil arrangement or a magnet, while it is further possible to generate the supporting field by means of a hard magnetic layer which is provided on or beside the MR elements.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.